Silicon nitride, sialon and similar ceramics characteristically not only have a smaller specific gravity and are more corrosive-resistant than steel but are also insulating. Accordingly, if ceramics are adopted as a material for a component of rolling bearings (including hub units) including a race member and a rolling element, such as a race member, and a rolling element, they can provide bearings and the like reduced in weight and also prevent rolling bearings from having short life as their components corrode and are thus damaged or electrolytically corrode.
Furthermore, a hub unit, which is a type of rolling bearing, is often used in an environment having a possibility of receiving moisture therein and hence having insufficient lubricity. Ceramic rolling elements, race members and similar rolling contact components are characteristically less damageable in such an insufficiently lubricating environment as above. Accordingly, for example, a hub unit with a rolling contact component formed with ceramics adopted as a material can exhibit improved durability when it is employed in an insufficiently lubricating environment.
Furthermore, if ceramics are adopted as a material for a component of a universal joint including a torque transmission member therefor, e.g., the torque transmission member therefor, they can provide a universal joint reduced in weight and also prevent the universal joint from having short life as a torque transmission member therefor corrodes and is thus damaged or electrolytically corrode.
Furthermore, a universal joint has a torque transmission member therefor rolling and stopping repeatedly on a surface of a race member, and between the torque transmission member for the universal joint and the race member there is not sufficient oil film provided. Furthermore, a universal joint is often used in an environment having a possibility of receiving moisture therein and hence having insufficient lubricity. A ceramic torque transmission member for a universal joint is characteristically less damageable in such an insufficiently lubricating environment as above. Accordingly, for example, a universal joint with a torque transmission member therefor formed with ceramics adopted as a material can exhibit improved durability when it is employed in an insufficiently lubricating environment.
Furthermore, if ceramics are adopted as a material for a sliding member that constitutes a plain bearing, a fluid dynamic bearing unit or a similar sliding device, and contacts another adjacent member and also slides relative to the other member, they can provide a sliding device reduced in weight and also prevent the sliding device from having short life as its sliding member corrodes and is thus damaged or a component electrolytically corrodes.
However, silicon nitride, sialon and similar ceramics require higher production cost than steel, and adopting ceramics as a material for components of rolling bearings, universal joints or sliding devices disadvantageously increases their production costs.
In recent years, there has been developed a method of producing β-sialon, a type of ceramics, inexpensively by adopting a production process including combustion synthesis (see Japanese Patent Laying-open No. 2004-91272 (Patent Document 1), Japanese Patent Laying-open No. 2005-75652 (Patent Document 2) and Japanese Patent Laying-open No. 2005-194154 (Patent Document 3) for example). This allows one to consider adopting β-sialon as a material for components of rolling bearings, universal joints or sliding devices to produce them inexpensively.
Patent Document 1: Japanese Patent Laying-open No. 2004-91272
Patent Document 2: Japanese Patent Laying-open No. 2005-75652
Patent Document 3: Japanese Patent Laying-open No. 2005-194154